Dear Editor:I read the article entitled “Histologic Evidence of New Collagen Formulation Using Platelet Rich Plasma in Skin Rejuvenation: A Prospective Controlled Clinical Study” published in the Annals of Dermatology1 with great interest. I would like to draw your attention to two important points.First of all, while platelet rich plasma (PRP) is prepared by differential centrifugation protocol, the force exerted on the blood sample varies depending on the diameter of the centrifuge: G-force or RCF (relative centrifugal force)= 1.12×radius of rotation in mm×(revolutions per minute [RPM]/1,000)2. Therefore, when talking about centrifugation, one should either use the term G-force or RCF, or if RPM is used, the radius of the rotor should be provided.Secondly, the total amount of growth factor and the efficiency of the treatment depends on the platelet concentration in PRP. Accordingly, the platelet count should be >1,000,000 platelets/µl for an effective treatment23. While the platelet concentration in PRP depends on the baseline platelet count of an individual, the device used, and the technique applied; it is not possible to achieve the desired PRP concentration if platelets are diluted with more than 10% of the blood sample. In the current study, however, the authors report that they obtained a PRP concentration of 2 ml (25%) from an 8 ml blood sample. Did the authors perform a platelet count from this sample to confirm that the sample was really platelet-rich?